This invention relates generally to detection systems and methods and, more particularly, to systems and methods for the detection of leaks from devices adapted to contain a pressurized fluid at a relatively high internal pressure, such as certain inflator devices used in the inflation of inflatable articles, such as an inflatable vehicle occupant restraint airbag cushions used in inflatable restraint systems.
It is well known to protect a vehicle occupant using a cushion or bag, e.g., an xe2x80x9cairbag cushion,xe2x80x9d that is inflated or otherwise expanded with gas when the vehicle encounters sudden deceleration, such as in the event of a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Upon actuation of the system, the cushion begins to be inflated, in a matter of no more than a few milliseconds, with gas produced or supplied by a device commonly referred to as an xe2x80x9cinflator.xe2x80x9d
The term xe2x80x9ccompressed gas inflatorxe2x80x9d is commonly used to refer to various inflator devices which contain a selected quantity of compressed gas. For example, one particular type of compressed gas inflator, commonly referred to as a xe2x80x9cstored gas inflator,xe2x80x9d simply contains a quantity of a stored compressed gas which is selectively released to inflate an associated airbag cushion. A second type of compressed gas inflator, commonly referred to as a xe2x80x9chybrid inflator,xe2x80x9d typically supplies or provides inflation gas as a result of a combination of stored compressed gas with combustion products such as result from the combustion of a gas generating material, e.g., a pyrotechnic.
In the past, stored gas inflators have generally been at a disadvantage, as compared to pyrotechnic inflators, in terms of size, weight, and/or cost. Such disadvantages have been especially significant in view of the general design direction of inflatable restraint systems toward relatively small, lightweight, and economical modern vehicle components and assemblies. In particular, the need in compressed gas inflators to store a gas at relatively high pressures typically results in the need for such an inflator device to include a pressure vessel having relatively thick walls. As a result, such vessels tend to be more bulky, heavy, and costly than otherwise desired for modern vehicle components.
Commonly assigned Smith et al., U.S. Pat. No. 5,470,104, issued Nov. 28, 1995; Rink, U.S. Pat. No. 5,494,312, issued Feb. 27, 1996; and Rink et al., U.S. Pat. No. 5,531,473, issued Jul. 2, 1996 disclose and relate to a recently developed type of inflator device, sometimes called a xe2x80x9cfluid fueled inflator.xe2x80x9d Such inflator devices typically utilize a fuel material in the form of a fluid, e.g., in the form of a gas, liquid, finely divided solid, or one or more combinations thereof, in the formation of an inflation gas for an airbag cushion. In one form of fluid fueled inflator, such a fluid fuel material is burned to produce gas which contacts a quantity of stored pressurized gas to produce inflation gas for use in inflating a respective inflatable device.
While such types of inflator devices can successfully overcome, at least in part, some of the problems associated with the above-identified prior types of inflator devices, there has been a continuing need and demand for further improved apparatus and techniques for inflating an inflatable device, such as an airbag cushion.
In at least partial response thereto, further efforts have led to the development of an apparatus for and methods of gas generation which at least in part rely on the decomposition or dissociation of a selected gas source material for gas generation. In particular, such developmental efforts have resulted in the development of an inflator device which is at least in part the subject of the above-identified patents: Rink, U.S. Pat. No. 5,669,629 and Rink et al., U.S. Pat. No. 5,884,938, as well as Rink et al., U.S. Pat. No. 5,941,562. In at least one form of such recently developed inflator device, inflation gas is produced or formed, at least in part, via the decomposition or dissociation of a selected gas source material, such as in the form of a compressed gas and such as via the input of heat from an associated heat source supply or device. Nitrous oxide is a gas source material disclosed for use in accordance with one or more of these patents. As disclosed, such an apparatus for and method of gas generation can be helpful in one or more of the following respects: reduction or minimization of concerns regarding the handling of content materials; production of relatively low temperature, nonharmful inflation gases; reduction or minimization of size and space requirements and avoidance or minimization of the risks or dangers of the gas producing or forming materials undergoing degradation (thermal or otherwise) over time as the inflator awaits activation.
In general, inflators have specific performance and operational requirements which necessitate that the inflators, or at least particular components thereof, be checked for the occurrence or presence of undesired leaks. For example, compressed gas inflators, such as described above, commonly require the presence therein of at least a certain specified quantity of the particular compressed material in order for the inflator to be capable of properly performing in the manner for which is was designed. In such inflators, it is generally desired that the amount(s) of stored compressed material(s) be maintained in the inflator within at least certain prescribed tolerances in order to ensure proper operation of the inflator. While proper inflator operation can be variously defined, ultimately, an inflator and the associated airbag cushion need provide adequate vehicle occupant protection over an extended period of time (typically fifteen years or more) after original construction and installation in a vehicle. Further, beyond the simple functioning of the inflator and deployment of the associated airbag, such inflatable restraint systems typically need to deploy the associated airbag cushion in a proper and particularly desired manner.
Various methods are available and have been used to determine the leak rate of compressed gas inflators. In practice, a typical or usual leak detection method involves the use of helium as a tracer gas included in the particular stored gas contents. In such a method, a certain fraction of the composition of the stored gas which escapes from the inflator consists of helium. (The exact fraction of helium detected as a result of a particular leak may be equal, less than, or greater than the corresponding loading conditions of the originally stored compressed gas. The physics associated with these various situations, however, is beyond the scope of the present discussion. In general, however, these different situations are typically dependent on certain, particular factors, such as the magnitude of the leak, the total pressure within the storage vessel, as well as the initial gas composition, for example.)
The leak rate of helium from a pressure vessel is normally detected using a mass spectrometer system. For such specific practice, mass spectrometers are normally designed to detect the presence of helium in the gases constituting the sample. The utilization of helium in leak tracing is advantageous in several respects: a) First, since the presence of helium is rather rare in the atmosphere, background helium (or residual helium in the environment such as the environment surrounding the detection apparatus) is normally very low. As a result, the possibility of the mass spectrometer being falsely influenced and possibly producing a spurious signal is significantly reduced or minimized; b) Second, the mass spectrometer signals for certain different molecular species can be nearly the same. Consequently, the mass spectrometer signal produced or resulting from the presence or occurrence of one molecular species may interfere or mask the mass spectrometer signal produced or resulting from the presence or occurrence of a different molecular species. For example, the molecular weights of nitrous oxide and carbon dioxide are approximately 44.02 and 44.01, respectively. As a result, it can be very difficult to distinguish between these molecular species via mass spectrometry. Helium, however, with a molecular weight of four, produces a mass spectrometer signal that is relatively easily distinguishable from the signal produced by other potentially present species; and c) Third, helium is a relatively small monatomic gas, facilitating the passage thereof through even relatively small or narrow leak paths.
Conventional helium leak detection techniques, however, suffer or potentially suffer from a number of problems or disadvantages. For example, in order to permit checking for leaks to the relatively small range of leakage acceptable in airbag module inflators, it is commonly necessary to include relatively large amounts of helium in the compressed gas mixture. In practice, the amount of helium required for inclusion is typically dependent on factors such as the magnitude and type of leak, the design life of the inflator, and the criteria for adequate performance for the inflator as a function of time. However, the incorporation of even moderate amounts of helium within a compressed gas inflator is or can be disadvantageous as, for a given volume, the storage pressure of the contents is significantly increased through such helium inclusion. Conversely, at a given pressure, the storage volume provided in or by the device needs to be increased in order to accommodate the mass of the added helium.
While the release of stored or included helium would normally also be expected to contribute to the inflation of the associated airbag, the inclusion and use of two or more dissimilar species (such as helium, which is normally a gas that will not liquefy and nitrous oxide, an easily liquefied gas) is especially problematic. For example, as separation of such species can easily occur, in practice it may be difficult to ensure a uniform mixture composition. As a result, the use of two or more molecular species commonly necessitates the use of additional storage, handling, and mixing equipment.
A significant limitation on such use of helium in such leak detection schemes is that the leak rate from a pressure vessel normally cannot be accurately checked at a date substantially later than the date the inflator is manufactured unless the helium concentration within the vessel is known. That is, unless the pressure vessel satisfies the limitations of either originally only containing helium or the leak is of the type that the compressed gases (e.g., both the primary stored gas and the helium tracer gas) are escaping in equal proportion to that at which they were loaded (as in the original composition), then the leak rate determination at such later points in time will normally be in error. The inflator assembly use of a pressure vessel originally only containing helium presents significant design limitations such as due to the typical bulkiness and mass associated with appropriate such pressure vessels. Further, as knowledge of the type of leak cannot be definitively known a priori, the making of the assumption that both the primary stored gas and the helium tracer gas are escaping in equal proportion to that at which they were loaded can result in significant error.
Other possible limitations or drawbacks to the use of such helium leak detection techniques include that the occurrence or presence of liquid materials within the storage vessel may impede the passage of helium through the leak or otherwise xe2x80x9cmaskxe2x80x9d the presence of the leak. For example, if a liquid with a relatively high surface tension is present in the vessel, such liquid could possibly flow into a hole through which gas would normally leak and may, at least temporarily, inhibit the passage of the gaseous leak trace material out of the inflator. However, with time, the liquid may no longer occupy the leak path and the stoppage of gas leakage therethrough may only be temporary.
In addition, though the occurrence or presence of helium in the general atmosphere is relatively rare, it will be appreciated that various manufacturing environments may produce, create, or have associated therewith relatively high background concentrations of helium. This may necessitate that a vessel to be tested be first isolated, such as by being placed in a closed chamber in which a vacuum is created in the surrounding environment, with the helium leak rate then being determined. Such special handling requirements can significantly add to the time and expense associated with the manufacturing process.
Further, the use of helium may undesirably result in the addition of considerable expense to the cost of the inflator, such as through the inherent cost of the helium itself, the cost of purchasing, calibrating, and maintaining the mass spectrometers, as well as the costs associated with the equipment required to store, mix, and handle the helium.
In view of the above, there has been a need and demand for a pressurized fluid-containing inflator design which facilitates leak detection. Further, there has been a need and demand for an inflator device which satisfies one or more of the following objectives: increased simplicity of design, construction, assembly, and manufacture; avoidance or minimization of the risks or problems associated with the storage, handling, and dispensing of gas generant materials; permits even further reductions in assembly weight and volume, or size; and realizes enhanced assembly and performance reliability.
At least partially in response to such needs and demands further efforts have led to the development of the apparatus for inflating an inflatable device and the methods of leak detection which are at least in part the subject of the above-identified patent: Rink et al., U.S. Pat. No. 5,884,938. For example, disclosed therein are embodiments utilizing cryogenically formed or frozen solid forms of gas source material. As disclosed, the inclusion and use of such cryogenically formed or frozen solid forms may facilitate and improve performance reliability, such as by minimizing or avoiding the appearance of possible leak paths in the resulting inflator devices. The inclusion of helium via a cryogenic or frozen solid form is generally not commercially practical nor realizable. Thus, in accordance with at least one embodiment disclosed in U.S. Pat. No. 5,884,938, the detection of the occurrence of a leak from an otherwise closed chamber which contains a pressurized fluid is accomplished through the inclusion of a selected quantity of a radioactive leak trace material within the chamber and then measuring the reduction or change in the radioactive signals emanating from the chamber.
While such a leak detection arrangement and method may successfully overcome, at least in part, some of the problems or shortcomings such as identified above with respect to conventional leak detection techniques and arrangements, there are continuing needs and demands for further improved systems and methods for the detection of leaks from devices adapted to contain a fluid at a relatively high internal pressure, such as certain inflator devices used in the inflation of an inflatable article, such as an inflatable vehicle occupant restraint airbag cushion used in inflatable restraint systems. In particular, there is a need and demand for systems and methods for the detection of leaks which avoid the need for inclusion of helium and which more effectively meet or satisfy one or more of the following objectives:
1. permits, facilitates, or is conducive to practice in a mass production environment,
2. allows or permits effective leak checking of a device or chamber at any selected point in time, including at a point in time substantially after manufacture or after return from the field,
3. can be applied to variously sized chambers or devices including very small chambers, such as chambers having storage cavities of 10xe2x88x923 cc or even smaller, and
4. avoids or otherwise eliminates concerns, such as relating to the inclusion of a radioactive material, albeit in very low concentrations or relative amounts, in a manufactured product or device.
A general object of the invention is to provide a unique system and method of leak detection.
A more specific objective of the invention is to overcome one or more of the problems described above.
The general object of the invention can be attained, at least in part, through a system for use in leak detection which system includes a pressure chamber adapted to hold at least one test object having a wall which contains contents at a pressure of at least about 200 psi (1.38 MPa). The system also includes a source of a tracer gas medium, such as includes at least one radioactive trace material. In accordance with one particular preferred embodiment, such trace material is desirably in the form of a radioactive isotope, such as Kr85, for example. The tracer gas medium source is in fluid communication with the pressure chamber whereby the tracer gas medium is externally applied to at least a portion of the wall.
The prior art generally fails to provide leak detection arrangements and methods for devices adapted to contain a fluid at a relatively high internal pressure, such as certain inflator devices used in the inflation of an inflatable article, such as an inflatable vehicle occupant restraint airbag cushion used in inflatable restraint systems and which arrangements and methods permit, facilitate, or are conducive to practice in a mass production environment, such as desired for economical practice. Further, the prior art generally fails to provide systems and methods for the detection of leaks from such devices or chambers at any selected point in time, including at a point in time substantially after manufacture. Still further, the prior art generally fails to provide systems and methods for the detection of leaks capable of effective practice with variously sized chambers or devices including very small chambers, such as chambers having storage cavities of 10xe2x88x923 cc or even smaller.
The invention further comprehends a method of leak detection. In accordance with one preferred embodiment of the invention, such a method of leak detection involves externally applying a first quantity of a tracer gas medium containing at least one radioactive trace material to at least a portion of a first chamber wall containing a pressurized fluid at a pressure of at least about 200 psi (1.38 MPa) of at least one test object and subsequently measuring the radioactive signals emanating from the first chamber. In practice, the tracer gas medium is desirably externally applied to the chamber wall at a pressure greater than the internal pressure of the fluid contained within the chamber. Again, in accordance with one particular preferred embodiment, such trace material is desirably in the form of a radioactive isotope, such as Kr85, for example.
As used herein, references to xe2x80x9ccombustion,xe2x80x9d xe2x80x9ccombustion reactionsxe2x80x9d, and the like are to be understood to generally refer to the exothermic reaction of a fuel with an oxidant.
References to xe2x80x9cdecomposition,xe2x80x9d xe2x80x9cdecomposition reactionsxe2x80x9d, and the like are to be understood to refer to the splitting, dissociation, or fragmentation of a single molecular species into two or more entities.
xe2x80x9cThermal decompositionxe2x80x9d is a decomposition controlled primarily by temperature. It will be appreciated that while pressure may, in a complex manner, also influence a thermal decomposition, such as perhaps by changing the threshold temperature required for the decomposition reaction to initiate or, for example, at a higher operating pressure change the energy which may be required for the decomposition reaction to be completed, such decomposition reactions remain primarily temperature controlled. Pressure may also cause one or more of the dissociative materials to liquefy. It should be understood or appreciated by one skilled in the art that liquefaction, with the associated changes and differences in vapor and liquid volumes, densities, and specific heats, as well as the introduction of the latent heat of vaporization, may also significantly influence the decomposition behavior.
xe2x80x9cExothermic thermal decompositionxe2x80x9d is a thermal decomposition which liberates heat.
The term xe2x80x9cequivalence ratioxe2x80x9d (xcfx86) is an expression commonly used in reference to combustion and combustion-related processes. Equivalence ratio is defined as the ratio of the actual fuel to oxidant ratio (F/O)A divided by the stoichiometric fuel to oxidant ratio (F/O)S:
xcfx86=(F/O)A/(F/O)S
(A stoichiometric reaction is a unique reaction defined as one in which all the reactants are consumed and converted to products in their most stable form. For example, in the combustion of a hydrocarbon fuel with oxygen, a stoichiometric reaction is one in which the reactants are entirely consumed and converted to products entirely constituting carbon dioxide (CO2) and water vapor (H2O). Conversely, a reaction involving identical reactants is not stoichiometric if any carbon monoxide (CO) is present in the products because CO may react with O2 to form CO2, which is considered a more stable product than CO.)
For given temperature and pressure conditions, fuel and oxidant mixtures are flammable over only a specific range of equivalence ratios. Mixtures with an equivalence ratio of less than 0.25 are herein considered nonflammable, with the associated reaction being a decomposition reaction or, more specifically, a dissociative reaction, as opposed to a combustion reaction.
Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawings.